This invention relates to a self-oscillating power-supply circuit for supplying a load with input voltages of different values and comprising a transformer with a primary winding and a secondary winding. The primary winding is in series with the main-current path of a switching transistor between two terminals to which the input voltage is to be applied. The switching transistor is turned off by means of a control transistor depending on a current proportional to the input voltage. The secondary winding is in series with a diode and this series combination is arranged in parallel with the load. A positive feedback loop comprising a capacitor in series with a first resistor is coupled between one end of the secondary winding and the base of the switching transistor.
Such a power-supply circuit may be used for charging batteries and powering electronic equipment. In particular, such a power-supply circuit may be employed in an electric shaver, in which the circuit supplies the charging current for the batteries and the current for the d.c. motor if the batteries are exhausted and the shaver is powered directly from the AC supply voltage.
A power-supply circuit of the type defined in the opening paragraph is disclosed in U.S. Pat. No. 4,504,775. If the input voltage is applied to this circuit a small current is fed to the base of the switching transistor via a starting resistor and drives this transistor partly into conduction. This results in a small primary current in the primary winding of the transformer. This current induces a voltage in the secondary winding, as a result of which a larger current is applied to the base of the switching transistor via the positive feedback between the secondary winding and the base of the switching transistor. In this way the switching transistor is rapidly driven into full conduction. During the so-called "forward" interval the primary current increases as a linear function of time. In the known circuit a resistor is arranged in the emitter line of the switching transistor, which resistor is connected in parallel with the base-emitter junction of a control transistor. For a specific value of the primary current the control transistor is then turned on and the switching transistor is consequently turned off. During the so-called "flyback" interval the energy stored in the transformer produces a secondary current in the secondary winding. This current constitutes the charging current for the batteries or directly powers the relevant apparatus. The seconary current decreases as a linear function of time until the switching transistor is turned on again.
In this way the power-supply circuit forms a self-oscillating power supply which supplies a constant average output current at a specific input voltage and a constant output voltage. However, this output current depends on the input voltage. For example, if the input voltage is higher, the primary current increases at a faster rate during the forward interval so that the level at which the switching transistor is turned off is reached more rapidly. Since the flyback interval is substantially constant, this leads to a higher oscillating frequency and hence to a larger average output current.
In addition, an increasing input voltage, as a result of the positive feedback between the secondary winding and the base of the switching transistor, leads to an increasing base current in this transistor. Increasing input voltages therefore drive the switching transistor further towards saturation so that after the primary current has reached the turn-off level the switching transistor is turned off with an increasing delay. As a result, the primary current becomes increasingly higher at increasing input voltages, which also results in an increasing average output current.
However, the output current of the power-supply circuit should remain within a specific range in order to preclude damage to the batteries and/or the apparatus as a result of an excessive current at high input voltages and in order to enable an adequate charging current for the batteries and/or power-supply current for the apparatus to be obtained at low input voltages.
To enable the power-supply circuit to be used with different AC supply voltages in various countries without adaptation or switching over, the known circuit employs a supply-voltage compensation by means of which the values of the primary current for which the switching transistor is turned off are reduced as the input voltage increases. This is effected, for example, by applying a current which is proportional to the input voltage to the resistor in the emitter line of the switching transistor. As a result the control transistor is turned on for values of the primary current which decrease as the input voltage increases.